UML Weekend Crash Course™ - To Parent Directory

168
Saturday Afternoon
Knowing the purpose of Sequence and Collaboration diagrams
There are actually three UML diagrams in the dynamic view: the Sequence diagram, the
Collaboration diagram, and the Statechart diagram. Sessions 16 through 19 cover the
Sequence and Collaboration diagrams, their notation and usage, and their similarities and
differences. Sessions 20 through 23 deal with the Statechart diagram.
The Sequence and Collaboration diagrams both illustrate the interactions between objects.
Interactions show us how objects talk to each other. Each time that one object talks to
another it talks to an interface (that is, it invokes an operation). So if you can model the
interactions, you can find the interfaces/operations that the object requires.
You may think it’s odd that the UML has two diagrams that do the same
thing. In a way, you’re right. The reason is that they came from two different
Tip methodologies and they each offer a slightly different perspective that can
be quite valuable. (You can find more on this in Session 18.)
In the Use Case view, you modeled the features of the system and developed scenarios
describing how the system should behave when those features are used. The Sequence
diagram provides a path from the textual descriptions of behaviors in the scenarios to
operations/interfaces in the Class diagram.
Mapping interactions to objects
Everything in an object-oriented system is accomplished by objects. Objects take on the
responsibility for things like managing data, moving data around in the system, responding
to inquiries, and protecting the system. Objects work together by communicating or interacting
with one another. Figure 16-1 shows a Sequence diagram with three participating
objects: Bill the Customer, Bill’s Order, and the Inventory. Without even knowing the notation
formally, you can probably get a pretty good idea of what is going on.
Steps 1 and 2: Bill creates an order.
Step 3: Bill tries to add items to the order.
Step 4 and 5: Each item is checked for availability in inventory.
Step 6 and 7: If the product is available, it is added to the order.
Step 8: He finds out that everything worked.
Building the Sequence diagram is easier if you have completed at least a first draft
of the Use Case model and the Class diagram. From these two resources, you get sets of
interactions (scenarios) and a pool of candidate objects to take responsibility for the
interactions.